The present invention relates to a mechanical seal for use in sealing rotation shafts such as those of liquid transfer pumps.
Each of mechanical seals used for sealing rotation shafts includes a rotation seal ring mounted on the rotating shaft and a stationary seal ring mounted on a casing, rotation and stationary seal rings having sealing end surfaces formed on their sides opposite to each other. The rotation seal ring, axially movable, is provided with a spring member which serves to urge the rotation seal ring toward the stationary seal ring. Therefore, the sealing end surface of rotation seal ring is slidably contacted with that of stationary seal ring by the action of the spring member and the pressure of liquid to be sealed. The width of sliding contact area between sealing end surfaces of seal rings is of several millimeters in this case.
However, it is difficult for the above-mentioned arrangement to accurately keep sealing end surfaces of rotation and stationary seal rings parallel to each other, said sealing end surfaces being slidably contacted with each other at the contact width of several millimeters. Therefore, it is common that only outer or inner circumference portions of sealing end surfaces are slidably contacted with each other because of pressure or heat distortion. It is also difficult to finish smooth sealing end surfaces each having a width of several millimeters in the radial direction thereof and a relatively wide area, and to keep the smoothness for a long period. Therefore, because of the face swelling of sealing end surfaces occuring in the radial direction thereof, sealing end surfaces are directly contacted with each other at a part thereof, while separated from each other at the other part thereof to form a relatively wide clearance therebetween. As a result, it becomes difficult to achieve an ideal sealing condition, that is, a condition under which sealing end surfaces are slidably contacted with each other with a thin liquid layer interposed therebetween, said liquid layer having a uniform thickness corresponding to several molecules. Said condition will be hereinafter referred to as the condition of boundary lubrication. Burning accidents will therefore be caused when the liquid layer interposed between sealing end surfaces becomes broken or too thin, while liquid leakage will increase when the liquid layer becomes too thick. Seal rings each having a sealing end surface whose width is large are necessarily made to form bulky block-shaped ones. When these seal rings are slidably contacted with each other, the heat generated between their sealing end surfaces is not easily dissipated into the liquid to be sealed, or the atmosphere and is likely to be stored in them. Accordingly, the temperature of sealing end surfaces becomes extremely high to make the lubrication bad, and causing burning accidents or abnormal wear of sealing end surfaces. These mechanical seals conventionally used for shaft sealing a variety of liquids are not yet employed in the fields of shaft sealing liquids which are most difficult to seal, said liquids including rubber-like ones such as latex which tend to coagulate under shearing friction, plastic ones copolymerized by sliding friction heat, petroleum ones such as heavy crude oil having high viscosity, black liquors in pulp industry, and mud water containing a large amount of slurry. The reason why conventional mechanical seals are not employed in the above-mentioned field is that a coagulated film of liquid to be sealed is formed and interposed between sealing end surfaces which are slidably contacted with each other over several-millimeters width and serves to separate sealing end surfaces from each other causing these sealing end surfaces to lose their sealing function in a matter of hours, and sometimes in minutes or seconds. The growth of this coagulated film can not be stopped even if each of sealing end surface is formed to have an extremely high smoothness below 1 .mu.m. In addition, conventional mechanical seals can not achieve satisfactory sealing function when the pressure of liquid to be sealed becomes inverse, high or large in change. This is because the spring face pressure to be imparted to the sliding contact area between sealing end surfaces of conventional mechanical seals is usually set to a relatively low value from 0.5 to 2 kg/cm.sup.2 (or from 7.11 to 28.44 lb/in.sup.2). As a result, when inverse pressure or the like is caused, sealing end surfaces can not resist the force of such inverse pressure and are separated from each other to form a wide clearance therebetween, thus increasing liquid leakage remarkably. Namely, the balance ratio S.sub.2 /S.sub.1 of conventional mechanical seals is usually in the range of from 0.7 to 1.2, wherein S.sub.1 represents the sliding contact area between sealing end surfaces and S.sub.2 that area of seal rings which is exposed to the pressure of liquid to be sealed. The lower balance ratio practically employed is of 0.65 and when it becomes lower than 0.5, no sealing function can be achieved theoretically. The pressure of liquid to be sealed gives great influence to the spring face pressure imparted to the sliding contact area between sealing end surfaces.